The fish may have escaped from a commercial salmon farm in British Columbia or Washington state.

Forest Service biologists found the Atlantic salmon last May while conducting a study of trout hybrids in the Martin River, which dumps into the Copper River Delta about 30 miles east of Cordova. The delta annually is the scene of Alaska's first major commercial salmon fishery.

The discovery of Atlantic salmon in Alaska waters, most commonly in Southeast, has disturbed Alaska fishermen who fear an alien takeover in Alaska's salmon spawning rivers. Although salmon and trout from foreign fish farms now dominate world markets once owned by Alaska's wild salmon, the state continues to outlaw fish farming and has decried neighboring British Columbia's recent decision to expand its farming industry.

"Clearly, we don't like it," Sue Aspelund, head of a commercial fishermen's association in Cordova, said of the Martin River discovery.

Gordie Reeves, a fish biologist from the U.S. Forest Service's Pacific Northwest Research Station in Corvallis, Ore., said biologists early last May were doing a study of cutthroat, rainbow and steelhead hybrids in the lower Martin, about half a mile up from the delta, when a member of the team landed an odd-looking catch on a spinner. It was only the second fish they caught that day, on the first day of the study and in the first fishing hole they tried.

"It was very silvery -- an adult fish," Reeves said. "The thing that struck us was the head shape, which was quite different. It was a bit more blunt or rounded. Also, the spotting pattern was different, like little x's across the back instead of round spots."

The fish came out of a pool that contained adult cutthroat trout. There was only clear, fresh water with no tidal influence at that point on the river, Reeves said.

The team decided to release the fish because its state research permit forbids killing any fish. But the biologists did take photos and a tail clipping, on which genetic tests confirmed it was an Atlantic salmon.

Reeves said he has little doubt the fish came from an aquaculture operation down south, where salmon are raised commercially in saltwater net pens. Mass escapes sometimes occur when the pens are damaged by storms or predators, although farmers say these "fish spills" have decreased.

Because the Martin River fish showed no signs of sexual maturity, biologists were not able to tell whether it was male or female.

"What it was doing in fresh water, boy, I wouldn't know," Reeves said.

At least two Atlantic salmon have been caught by commercial fishermen in the Sound, one in 1992 in Port Valdez, and another in the saltwater of the Copper River Delta in 1998, Miller said.

According to the Alaska Department of Fish and Game, the first Atlantic salmon was recovered in Southeast waters in 1991 and since then almost 600 have been documented, nearly all in Southeast.